Method of producing metallic powders



United States Patent METHOD OF PRODUCING METALLIC POWDERS Joseph J.Wimberly, College Park, Ga., assignor to Tennessee Corporation, NewYork, N. Y., a corporation of New York No Drawing. Application October29, 1952,

Serial No. 317,573

4 Claims. (Cl. 75-.5)

This invention relates to the production of powdered metals, forexample, powdered metallic iron, for use in powder metallurgy. Y

Powder-metallurgy processes for producing various types of metallicarticles and products are now well known. The metal powders heretoforeused have been produced by several different methods, includingelectrolysis, spraying or atomization, reduction of metal oxides, andother chemical methods. In general, metallic articles are manufacturedfrom such powders by compressing the powder to the desired shape in asuitable mold and then subjecting it to an appropriate heat treatment.Such procedures are employed for a variety of purposes such as theproduction of machine parts and bearings, the manufacture of variousarticles from special alloys, and many others.

For best results in powder-metallurgy processes, the meshclassification, particle size distribution, uniformity and apparentdensity or apparent specific gravity of the powder must be appropriatelycontrolled within well defined limits. Accordingly powders produced bythe methods mentioned above are usually pulverized, screened and blendedto assure that they have the desired properties before usev For example,an iron powder to be acceptable for most purposes must conform to thefollowing requirements:

1. It must have an apparent specific gravity between 2.2 and 2.5.

2. It must have a flow rate of 40 seconds or less, as determined by thetime required for 50 grams to pass through a Hall flowmeter.

3. All of the material must be less than 100 mesh in particle size, anda maximum of 60% may be less than 325 mesh. Within these limits, theparticle size distribution or gradient may vary somewhat, but must beapproximately of the order of one third greater than 200 mesh, one thirdgreater than 325 mesh, and one third less than 325 mesh.

Oftentimes it is very difficult to meet such requirements, particularlyin the case of metal powders obtained by reduction of metal oxides bymeans of gaseous or solid reducing agents. Even when ground to thepermissible limit from the standpoint of particle size distribution, forexample, the apparent specific gravity of the reduced and ground powdermay still be less than the acceptable minimum and the powder thereforerelatively worthless for the purposes of the present invention. In othercases the apparent specific gravity may fall within prescribedacceptable limits, but the powder may have an unacceptable particle sizedistribution; for example, it may contain too large a percentage ofsmall particles.

The present invention provides improved methods of handling and treatingsuch metal oxides, in particular prior to the reduction thereof, so asto minimize and in many cases to eliminate entirely the difiicultiesmentioned above and to facilitate the production of metal powdersmeeting the specifications prescribed for powder metallurgy.

2,7Zl,l35 Patented Oct. 18, 1955 The invention has as a further objectto make it possible to predetermine the characteristics of the ultimatemetal powder by adjustment of the nature and properties of the metaloxide prior to reduction.

A still further object is to provide an improved method for treatingpulverulent metal oxides having a particle size gradient such thatheretofore they have been considered unsuitable for the manufacture ofmetal powders, Whereby such oxides can be reduced and ground to providepowders completely satisfying the accepted specifications for powdermetallurgy.

Another object is to provide an improved metal powder capable ofproducing metallic articles of increased tensile strength as comparedwith metal powders heretofore used.

Still another object is to facilitate the reduction of the oxide.

I have found that the foregoing objects can be achieved by sintering themetallic oxide and then crushing and/ or grinding the sintered materialto the desired mesh size before reduction. Sinter particles, even ofsizes down to one micron or less, are characterized by a porous cellularstructure resembling that of coke or a sponge. I

have observed that when such oxidic sinter particles are reduced at atemperature below the fusion point, the resulting metallic particleshave the same cell-like structure and approximately the same size andirregular shape as the original sinter particles.

Thus by first sintering the oxidic material from which the powder is tobe made, and by other known techniques such as screening, crushing andgrinding, a particular oxidic sinter can be produced havingapproximately the particle size distribution desired in the finalpowder, and particle size will be approximately maintained duringreduction so that further grinding or the like after reduction producesa metal powder having about the same particle size distribution as thesinter prior to reduction. This can be accomplished even with oxidicmaterials so finely divided that they have not been useful for powdermetallurgy heretofore, since the necessary particle size enlargement isone of the results of sintering.

Any suitable sintering method and apparatus can be employed, preferablythe well known down-draft Dwight- Lloyd or Greenawalt sintering machinesin which a mixture of the oxidic material with fuel and water is burnedon a down-draft sintering pan. Small particles of sinter can be screenedout and used in the next batch of sinter. The oxidic sinter is thencrushed and/or ground in any suitable manner to produce a particulatesinter having a mesh classification and particle size distributionwithin the limits desired in the final powder and then reduced by meansof solid carbon or a gaseous reducing agent such as hydrogen, carbonmonoxide, and mixtures of the same. Since some agglomeration takes placeduring reduction, the reduced material is then reground or milled untilit once more has the desired predetermined particle size distribution asexplained above.

Moreover, it has been found that the porous cellular structure of thesinter particles greatly facilitates the complete reduction of the oxidein a short time. Another advantage is that the reduced particles havethe same irregular shapes and sizes as the sinter particles and theresulting interlocking qualities increase the tensile strength of thepressed parts made from the powder.

Also the sintering step burns out impurities such as sulfur; thereforethe starting material may be a sulfide since it will be converted to anoxide in the sintering operation.

Accordingly it will be understood that the invention is applicable toany metal-bearing material which, after sintering, comprises an oxidecapable of being reduced without volatilizing the metal, includingmetallic ores, mill scales, etc. Its advantages are well illustrated byits apin a typical case:

plication to a magnetite ore containing excess silica impurity asfollows:

In order to reduce the silica content of the ore to below 0.2 of one percent by magnetic separation, it is necessary to grind the ore to thefollowing mesh sizes:

7 Per cent 100 mesh 200 mesh Trace 325 mesh 10.0 325 mesh 90.0

This material can be reduced to metallic iron and ground or milled tobring its apparent specific gravity within the required limits, but thesize gradient is not ac- Accordingly the ground ore after separation ofsilica was sintered by burning a mixture of ore, coke, and water on adown-draft sintering pan similar in operation to a Dwight-Lloyd orGreenawalt sintering machine which are familiar to those versed in theart of sintering. The sintered ore varied. from large lumps, 6" or moreacross, down to fine particles. Any particles under 4 mesh were screenedout and put into the next batch of sinter.

The sinter was then crushed down to A1" size or smaller, and then groundto the desired size gradient. For ex-' ample, the following particlesize gradient was obtained Per cent On -l00 mesh 0 On 200 mesh 43.4 On325 mesh 19.0 Thru 325 mesh- 37.6

This ground sintered ore was reduced and then milled or ground until theapparent specific gravity was within the desired range. This gave aniron powder with the follow- It is evident that the abovecharacteristics are well within the accepted limits and in fact thereduced powder was highly satisfactory, whereas the powder obtained byreduction of the ore without sintering was not acceptable.

It will be noted that the particle size gradient of the reduced powderwas approximately the same'as'that of the Thus by pregrinding a groundsinter'before reduction. sintered material to a particular sizegradient, the corresponding properties of the resulting reduced andreground material can be predetermined and controlled. This etfect andresult are further illustrated by the following examples:

A screen analysis on' a ground sintered ore showed the particle sizegradient to be:

Per cent 7 On 100 mesh"; 0

On 200 mesh 32.2 On 325 mm.- 21.6 Thru 325 mesh 46.2

When'this ore was reduced and milled, an iron powder 7 with thefollowing characteristics was obtained;

. 4 Apparent specific gravity 2.04 Flow rate;; seconds 38.8

Particle size gradient:

On 100 mesh percent 0 On 200 mesh d0 23.6 On 325 mesh do 27.6 Thru 325mesh do 48.8

It will be seen that although near the limit with respect to particlesize gradient, this powder still was not acceptable because of its lowspecific gravity. The relatively large percentage of very smallparticles in the ground sinter was practically duplicated in the ironpowder, and

prevented further grinding in an efiort to improve the apparent specificgravity.

Another illustration of this effect was noted in the case of a coarseground sintered ore having a particle size gradient as follows:

Per cent On 100 mesh 0 On 200 mesh 52.8 011 325 mesh 18.2 Thru 325 mesh29.0

When the reduced material was milled to Within the desired specificgravity range the results were:

Apparent specific gravity 2.44 Flow rate seconds 28.8

Particle size gradient:

On 100 mesh per cent 0 On 200 mesh do 35.0 On 325 mesh do 29.6 Thru 325mesh do 35.4

It will be seen that this material is just acceptable as to its apparentspecific gravity and that a slightly coarser ground sinter would haveproduced a powder that was too heavy.

Thus by sintering and pregrinding the sinter before reduction, it ispossible to regulate approximately the physical. properties that theresulting metallic powder will have. By sintering, moreover, it ispossible to effect an initial size enlargement of excessively finelyground ores and like materials, so that materials heretofore consideredunsuitable for powder metallurgy can be prepared for such pregrindingand reduction to metal powders of high quality. At the same timereduction of the oxide is facilitated; burnable impurities such assulfur are removed; homogeneous metallic particles of porous or cellularstructure are obtained, thus providing a uniform material so that thepowder'metallurgy process can be controlled withpredictable andreproducible results; and

1. In the production of metal powder for use in powder V metallurgy froma finely divided metal-bearing material which when'reduced to metalforms a powder of particle size smaller than desired for such use, themethod which comprises the steps of sintering said metal-bearingmaterial to a metallic oxide with concomitant particle size enlargement,then comminuting the sinter to the particle size distribution desiredfor said metal powder, then reducing the oxide, and finally breaking upagglomerates formed during the reduction step.

2. The method of preparing a metal powder for use in powder metallurgyfrom an oxidic ore containing silicalike impurities which comprisescomminuting the ore to separate said impurities with concomitantreduction of particle size below that desired in said powder, thensintering said comminuted material to a metallic oxide with concomitantparticle size enlargement, comminuting the sinter to a particle sizedistribution Within the limits required for the metal powder, thenreducing the comminuted sinter, and recomminuting the reduced materialto a final particle size distribution within said limits.

3. The method of preparing an iron powder for use in powder metallurgyfrom an iron oxide ore containing silica which comprises finely dividingthe ore to detach silica from the ore particles with concomitantreduction of particle size below that desired in said powder, thenseparating and sintering said comminuted ore with concomitant particlesize enlargement, comminuting the sinter to a particle size distributionwithin the limits required for the metal powder, then subjecting thecomminuted sinter to heat in the presence of a reducing agent, andrecomminuting the reduced material to less than 100 mesh and at least40% greater than 325 mesh.

4. The method defined in claim 3, applied to magnetite ore.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN THE PRODUCTION OF METAL POWDER FOR USE IN POWDER METALLURGY FROM AFINELY DIVIDED METAL-BEARING MATERIAL WHICH WHEN REDUCED TO METAL FORMSA POWDER OF PARTICLE SIZE SMALLER THAN DESIRED FOR SUCH USE, THE METHODWHICH COMPRISES THE STEPS OF SINTERING SAID METAL-BEARING MATERIAL TO AMETALLIC OXIDE WITH CONCOMITANT PARTICLE SIZE ENLARGEMENT, THENCOMMINUTING THE SINTER TO THE PARTICLE SIZE DISTRIBUTION DESIRED FORSAID METAL POWDER, THEN REDUCING THE OXIDE, AND FINALLY BREAKING UPAGGLOMERATES FORMED DURING THE REDUCTION STEP.